It is common practice to form the base layer of a bipolar transistor through epitaxial deposition. This did not present serious problems in early devices but, as the sub-structure beneath the device has grown more complicated, and dimensional tolerances have narrowed, it has become necessary to develop methods for growing a base layer inside a small preset window and to then be able to make satisfactory contact to it. The selective epitaxial base bipolar transistor can allow the scaling of bipolar devices to less than that obtainable with conventional ion implantation techniques to produce a narrow base width in the deep submicron region.
A widely used approach to solving this problem has been selective epitaxial growth (SEG). SEG is a process that deposits single crystal Si layers only on the exposed Si substrate surface within the opening in the dielectric mask film, without the simultaneouds growth of Si in any form (polysilicon or amorphous) deposited on the silicon dioxide or silicon nitride layer. In the SEG process, SiCl.sub.4 gas has frequently been used for the silicon source. The addition of HCl (or Cl.sub.2) to the reactive gas is believed to increase the selectivity of the growth process. Many factors may affect the selective nature of Si deposition, including the Si substrate surface condition, dielectric opening size, HCl concentration, silicon source, growth pressure and growth temperature. Many defects (e.g. microtwins, stacking faults and dislocations), can be observed at or near the epitaxi/dielectric interface.
The present invention teaches how non-selective deposition can be used to achieve the same end result as the prior art SEG process. The aforementioned crystal defects can be greatly reduced in the new process.
A routine search of the patent literature was performed but no references teaching the process of the present invention were encountered. Several references of interest were, however, identified. For example, Herbert et al. (U.S. Pat. No. 5,773,350) teach the formation of a base layer for a bipolar transistor through epitaxial growth but polysilicon contacts to said layer are formed separately and are not self aligned.
Sato (U.S. Pat. No. 5,599,723) grows his base layer through a mask using epitaxy. The mask opening is intentionally over-etched so as to produce an overhang at its edges. This eliminates any possibility of silicon deposited during the same step being used as a butted contact to the base layer.
Kimura et al. (U.S. Pat. No. 5,614,425) also deposits a silicon layer that is partly epitaxial (monocrystalline) and partly polycrystalline. They then mask the monocrystalline portion with insulation and proceed to use selective epitaxy in order to deposit additional silicon on the polycrystalline portion.